The invention disclosed herein relates to position determining devices, systems and methods, particularly to digitizer systems of the type including a tablet having a conductor structure which interacts with a movable element or pointer such as a stylus or cursor and the like to provide signals, particularly for a computer, representing the position of the movable element relative to the tablet.
Application Ser. No. 07/505,944, the disclosure of which is hereby incorporated herein by reference, addressed the need for digitizer systems and digitizer conductor structures which employ a reduced number of connections between the conductor structure and the signal processing circuitry, which utilize conductor patterns that may be determined relatively easily, which locate a movable element with respect to two conductors with simplified processing, and which perform, for example, with acceptable resolution and accuracy.
This application addresses the same needs and discloses a digitizer system and conductor structure which are improvements in certain respects over those disclosed in Ser. No. 07/505,944.
According to Ser. No. 07/505,944, a conductor structure or system (sometimes referred to as a "grid") includes, for each axis, a number of conductors, at least one of which is run in a serpentine path with a non-uniform repeat increment. A conductor run in such a serpentine path includes for a given axis spaced active portions or runs which are substantially parallel to each other running substantially parallel to the axis, and connecting portions interconnecting or "spanning" the active portions. The active portions of the respective conductors may be connected in series, in parallel or in other ways. The repeat increment or "span" is simply the spacing between one run and the next of the same serpentine conductor, and may (but need not, depending on factors such as noise immunity and fine position determination accuracy) be constrained by a maximum repeat increment, and/or a minimum repeat increment, and/or a maximum change in repeat increments between consecutive runs of a same conductor. Ser. No. 07/505,944 describes grids laid out to provide codes which change in only one bit from state-to state, with no significance to each bit and no least or most significant bit. An algorithm for generating such a code, subject to certain constraints, is disclosed in Ser. No. 07/505,944. Ser. No. 07/505,944 also discusses potential inaccuracies which could occur when the repeat increments on each side of the conductor runs being used for fine interpolation are unequal.
Arranging conductors in a pattern so that spaces or regions between conductor runs (or the runs themselves) may be uniquely identified by unique binary numbers, e.g., in a Gray-type code, are not new developments. See, for example, U.S. Pat. Nos. 3,466,646 of Lewin, issued Sep. 9, 1969; 3,647,963 of Bailey, issued Mar. 7, 1972, assigned to the assignee of this application; 3,705,956 of Dertouzos, issued Dec. 12, 1972; 3,819,857 of Inokuchi, issued Jun. 25, 1974; 4,029,899 of Gordon issued Jun. 14, 1977; 4,378,465 of Green et al., issued Mar. 29, 1983; 4,552,991 of Hulls, issued Nov. 12, 1985; 4,734,546 of Landmerer, issued Mar. 29, 1988; 4,831,216 of Landmerer, issued May 16, 1989; and 4,835,347 of Watson, issued May 30, 1989.
One approach in digitizer systems employing serpentine-type grid conductors was to successively divide the active area into smaller and smaller regions. Thus, for example, one conductor divided the tablet in a given axis in half, another conductor in quarters, another in eighths, etc. With three conductors, a tablet may be divided according to this approach into eighths, with four conductors into sixteenths, with five conductors, into thirty-seconds. See, for example, the Inokuchi '857 Patent.
Grid conductors laid out in the classic Gray Code sequence disclosed in the Dertouzos '956 and Inokuchi '857 Patents introduce several sources of inaccuracy and are severely limited in feasible grid sizes, because the classic Gray code sequence has a least significant bit (LSB) which changes often (i.e., has many repeating runs), and a most significant bit (MSB) which changes rarely (i.e. has few repeating runs, typically just one which bisects the entire grid or tablet area). Thus, the MSB conductor divides the active area in half, which for a 12 inch tablet separates the adjacent active portions or runs of that conductor by six inches, and for a 24 inch tablet, separates the active portions of the conductor dividing the tablet in half by 12 inches. To operate with large spaces between conductor active portions of the MSB conductor while providing unambiguous signals induced in either the grid conductors or the movable element conductor, requires one or more of the following: high signal levels; a large conductor in the movable element; or sensitive processing circuitry. To avoid such requirements and to allow the bit place represented by a given conductor, e.g., the MSB conductor, to be read unambiguously for accurate coarse position determination, the total distance between active portions of a conductor is not made greater than a given distance, e.g., twice the "range" of the pickup device, as determined by signal strength and noise.
Since in such classic Gray code sequence grids the length of the LSB conductor is significantly longer than the MSB conductor, and since the linear relative lengths of other conductors are also different, some form of compensation, e.g., in processing, and/or different drives and/or different amplifications is required for each conductor to maintain signal levels in a proper relationship. Another possible source of error in such classic Gray code sequence grids stems from the differences in responses from conductors with different repeat increments. For example, the response to/from the LSB conductor will be affected by the close repeated runs, while the MSB conductor, not being repeated, will not be so affected, and the conductors between these extremes will be affected to different degrees as the number of runs of a conductor goes from 2, 4, 8, 16, etc. Thus, an interpolation of signal values in the region between the conductor runs having large differences in repeat increments, e.g., the MSB and LSB conductors, will be inaccurate due to these different characteristics.
Another approach was to divide the tablet area into halves, quarters, etc., as above described, but using a plurality of runs of the same conductor in each sub-divided portion of the tablet, rather than only one conductor run per tablet half, tablet quarter, etc. See the Lewin '646 Patent cited above. Since in this approach the individual runs of the same conductor are not widely separated, the digitizer system does not require the higher signal levels, larger movable element conductor or sensitive processing circuitry of the approach discussed above. However, with a given spacing between adjacent conductor runs of all conductors (said given spacing being referred to herein as "basic spacing"), using a number of runs of the same conductor in the same sub-divided tablet portion simply reduces the total number of unique spaces that can be identified with a given number of conductors.